I've seen some people hold Arya responsible over what happened to her friend Mycah at the Trident incident because according to them "she should have known her place in society and the power a lady holds over commoners". I disagree with this notion on so many levels.
First of all, Arya is only nine years old during that incident . Even if you believe that she shouldn't befriend commoners (which in my opinion she's totally allowed to, and I'll express my thoughts on it later on) shouldn't you be less harsh on the judgement of such a little kid? Even if according to you "she doesn't know her place in society", she still have plenty of years before she reaches adulthood to find the so called place she is supposed to occupy as a lady in Westerosi society.
As I said earlier on, I do believe that Arya is totally justified in befriending people who belong in a different social class from her. That's isn't only my own personal belief as someone who is anti - classism and lives in 21st century, but this is also supported by the text.
From her first introduction we know that Arya likes to befriend all sort of people.
Arya had loved nothing better than to sit at her father's table and listen to them talk. She had loved listening to the men on the benches too; to freeriders tough as leather, courtly knights and bold young squires, grizzled old men-at-arms. She used to throw snowballs at them and help them steal pies from the kitchen. Their wives gave her scones and she invented names for their babies and played monsters-and-maidens and hide-the-treasure and come-into-my-castle with their children. Fat Tom used to call her "Arya Underfoot," because he said that was where she always was. She'd liked that a lot better than "Arya Horseface."
The above passage is from the second chapter of hers, before she left Winterfell. She loves Winterfell's small folk and is loved by them in return. She is even given the nickname " Arya Underfoot" because of that behavior of hers. So, it's no secret that she associates with commoners. It's impossible for her mother and her father not to know. And yet she's never scolded for that, and Arya is scolded over plenty things by her mother and the Septa but never about the people she chooses as her companions.
It makes sense that she's allowed to associate with these people, since Bran in his own POV also expresses fondness for people who belong to a lower class (and he's also never forbidden to associate with these people) and Ned, the Winterfell's own Lord, is known to dine with people who belong to a lower class. So, it's totally okay for Arya - and for any other child of his- to follow his example.
Also, post the Trident incident, Ned has a long and serious talk with Arya. If he believed that his daughter shouldn't befriend a boy from a below class, he would express this opinion of his to Arya. But he didn't, because he didn't find anything wrong with it
The only people who find wrong Arya associating with small folk are Sansa and the Lannisters/Baratheon. The first is the only person who actually shares her distaste for Arya's company in her own POV:
Sansa knew all about the sorts of people Arya liked to talk to: squires and grooms and serving girls, old men and naked children, rough-spoken freeriders of uncertain birth. Arya would make friends with anybody. This Mycah was the worst; a butcher's boy, thirteen and wild, he slept in the meat wagon and smelled of the slaughtering block. Just the sight of him was enough to make Sansa feel sick, but Arya seemed to prefer his company to hers.
But we shouldn't take Sansa's view as the norm for westerosi society and especially not for Winterfell's household since her own father is okay with associating with people that belong to a lower class than theirs.
I believe that since Sansa is introduced to us as a proper little lady, some people take her views as the absolute truth when it comes to westerosi etiquette But they forget that Sansa is also a little kid who doesn't fully understand or see eye to eye with her little sister and therefore it makes sense that she views Arya's actions in a more negative light than other characters do.
As for the Lannister-Baratheon loyal family, Cersei and Joffrey have a strong distain for small folk and believe in their own superiority - but they also believe in their superiority over their fellow noble people. I guess we could say that both suffer from superiority complex and have a distorted idea of the world, so I wouldn't hold their own views as the norm, either.
I'm not saying that there aren't other nobles who believe in their own superiority over commoners and would never befriend people from a lower class, because the books contain plenty of these type of characters. I'm just saying that this isn't the absolute truth to every single noble character, aside from Arya. The kids in Winterfell are allowed to befriend people from lower classes and so are the Martell kids in the Water Gardens.
And not every monarch values so little the life of their people that they would order a little boy to be killed just because their child and crown prince, threw a fit. Ask yourself the question: if Ned Stark was the King and Robb had terrorised a little boy who played with Arya/any other little noble kid, would Ned order Mycah's death? Or would he have a long talk with his heir on how he shouldn't treat his people as objects? Just because Cersei and Joffrey don't give a fuck for small folk ( and Robert could not be bothered to interfere) it doesn't mean that every monarch would react the same way they did.
Mycah died because of Cersei and Joffrey cruelty and Robert's indifference. Not because Arya befriended him.
